There are a large number of antenna transmission schemes in Long-Term Evolution (LTE). The network node may use any of the different antenna schemes available in cells. Furthermore the antenna schemes may also be different in different cells. The network may also transmit the radio signals used for positioning measurements by the User Equipment (UE) via more than one antenna in a cell. The positioning measurement such as Observed Time Difference of Arrival (OTDOA) Reference Signal Time Difference (RSTD) is performed in several cells (e.g., 16 cells). However the UE is not aware of the antenna scheme used in neighboring cells. This in turn degrades the positioning measurement performance and may even lead to failure of positioning.
Positioning Overview
Several positioning methods for determining the location of the target device, which can be a UE, mobile relay, PDA (Personal Digital Assistant), etc. exist. Known methods include:                Satellite based methods, which use A-GNSS (e.g., Assisted Global Navigation Satellite System, Assisted Global Positioning System, A-GPS, etc.) measurements to determine UE position;        OTDOA, which uses UE RSTD measurement to determine UE position in LTE;        UTDOA (Uplink Time Difference of Arrival), which uses measurements done at LMU to determine UE position;        Enhanced cell ID, which uses one or more of UE Rx-Tx (Receive-Transmit) time difference, BS (Base Station) Rx-Tx time difference, LTE P/RSRQ, HSPA (High Speed Packet Access) CPICH (Common Pilot Channel) measurements, angle of arrival (AoA), etc. for determining UE position; and        Hybrid methods, which use measurements from more than one method for determining UE position.        
In LTE, the positioning node (also known as E-SMLC, Evolved Serving Mobile Location Center, SLP, or Secure User Plane Location (SUPL) Location Platform, or location server) configures the UE, eNodeB, or LMU (Location Measurement Unit) to perform one or more positioning measurements. The positioning measurements are used by the UE, positioning node (also referred to as a positioning server), or another node to determine the UE location. The positioning node communicates with UE and eNodeB in LTE using LPP (LTE Positioning Protocol) and LPPa protocols.
Positioning Architecture in LTE
The three key network elements in an LTE positioning architecture are the LCS (Location Services) Client, the LCS target, and the LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. A LCS Client is a software and/or hardware entity that interacts with a LCS Server to obtain location information for one or more LCS targets, (i.e., the entities being positioned). LCS Clients may also reside in the LCS targets themselves. An LCS Client sends a request to LCS Server to obtain location information, and the LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from the terminal or a network node or an external client.
Position calculation can be conducted, for example, by a positioning server (e.g., typically E-SMLC or SLP in LTE, although there is also a possibility to configure other nodes as positioning servers) or the UE. The former approach corresponds to the UE-assisted positioning mode, while the latter corresponds to the UE-based positioning mode.
Two positioning protocols operating via the radio network exist in LTE, LPP and LPPa. The LPP is a point-to-point protocol between a LCS Server and a LCS target device, used to position the target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel thereby reducing latency. LPPa is a protocol between eNodeB and LCS Server specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. SUPL (Secure User Plane Location) protocol is used as a transport for LPP in the user plane. LPP also has a possibility to convey LPP extension messages inside LPP messages, e.g., OMA (Open Mobile Alliance) LPP extensions (LPPe) to allow, e.g., for operator-specific assistance data or assistance data that cannot be provided with LPP or to support other position reporting formats or new positioning methods.
A high-level architecture, as it is currently standardized in LTE, is illustrated in FIG. 1, where the LCS target is a terminal, and the LCS Server is an E-SMLC or an SLP. In the figure, the control plane positioning protocols with E-SMLC as the terminating point are shown as LPP, LPPa, and LCS-AP, and the user plane positioning protocol is shown as SUPL/LPP and SUPL. SLP may comprise two components, SPC (SUPL Positioning Center) and SLC (SUPL Location Center), which may also reside in different nodes. In an example implementation, SPC has a proprietary interface with E-SMLC, and Llp interface with SLC, and the SLC part of SLP communicates with P-GW (PDN-Gateway) and External LCS Client.
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.
OTDOA Positioning
The OTDOA positioning method makes use of the measured timing of downlink signals received from multiple eNodeBs at the UE. The UE measures the timing of the received signals using assistance data received from the LCS server, and the resulting measurements are used to locate the UE in relation to the neighbouring eNodeBs.
With OTDOA, a terminal measures the timing differences for downlink reference signals received from multiple distinct locations. For each (measured) neighbor cell, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between neighbor cell and the reference cell.
The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias. In order to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed.
To enable positioning in LIE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning (positioning reference signals, or PRS [3GPP TS 36.211, V10.3.0, September 2011, 103 pages]) have been introduced and low-interference positioning subframes have been specified in 3GPP (3rd Generation Partnership Project).
PRS are transmitted from one antenna port (R6) according to a pre-defined pattern [3GPP TS 36.211]. A frequency shift, which is a function of Physical Cell Identity (PCI), can be applied to the specified PRS patterns to generate orthogonal patterns and model the effective frequency reuse of six, which makes it possible to significantly reduce neighbour cell interference on the measured PRS and thus improve positioning measurements. Even though PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g., cell-specific reference signals (CRS) could in principle also be used for positioning measurements.
PRS (positioning reference signals) are transmitted in pre-defined positioning subframes (e.g., having a period of N subframes) grouped by several consecutive subframes (NPRS), i.e., one positioning occasion (including NPRS=6 consecutive subframes) as shown in FIG. 2 (illustrating a positioning subframe allocation in time for a single cell). Positioning occasions occur periodically with a certain periodicity of N subframes, i.e., the time interval between two positioning occasions. The standardized periods N are 160, 320, 640, and 1280 ms, and the number of consecutive subframes may be 1, 2, 4, or 6 [3GPP TS 36.211].
Multi-Antenna Systems
The multiple input multiple output (MIMO) technique is an advanced antenna technique to improve the spectral efficiency and thereby boost the overall system capacity. MIMO implies that both the base station and the UE employ multiple antennas. MIMO techniques are widely studied and applied in practice for downlink communications, i.e., from the base station to the mobile terminal. Several MIMO techniques which are well-known and used in practical systems are explained below.
Irrespective of the MIMO technique the notation (M×N) is generally used to represent MIMO configuration in terms of number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (8×4). The configurations represented by (2×1) and (1×2) are special cases of MIMO that correspond to transmit diversity and receiver diversity, respectively. The configuration (2×2) will be used in WCDMA release 7.
The Evolved UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network (E-UTRAN) downlink will indeed support several MIMO schemes including MIMO techniques including Single-User MIMO, SU-MIMO, and Multi-User MIMO, MU-MIMO.
The MIMO technology has also been widely adopted in other wireless communication standards, e.g., IEEE802.16.
The above mentioned MIMO modes or other MIMO techniques enable some sort of spatial processing of the transmitted and received signals. This ability of spatial diversity in general improves spectral efficiency, extends cell coverage, enhances user data rate, mitigates multi-user interference, etc. In essence each MIMO technique has its own benefit. For example, receiver diversity (1×2) may improve coverage. On the other hand (2×2) MIMO (such as D-TxAA) may lead to increase peak user bit rate.
In general, a (2×2) MIMO scheme may double the data rate. The possibility to double the data rate depends on whether the channel is sufficiently uncorrelated so that the rank of the (2×2) MIMO channel matrix is 2 (the rank is the number of independent rows or columns of the matrix). In general, with (2×2) MIMO the average data rate will be lower than 2 times the data rate achieved in single link conditions.
Different possible multi-antenna techniques can be applied, for example, beamforming or antenna switching. Depending on whether the receiving eNodeB is equipped with multiple receiving antennas, transmit diversity (2 transmit antennas, 1 receiving antenna) or MIMO (2×2) will be discussed. Moreover, the scheme can be open loop or closed loop. Open loop multi-antenna techniques are based on the assumption that the base station (BS) does not have information about the downlink, DL channel, so that the base station cannot exploit this knowledge in order to improve/optimize the transmission weights (the transmission beamforming) to steer the beam in the direction of the UE. On the contrary, in case of closed loop multi-antenna techniques, the BS has some information about the DL channel which it can exploit to optimize/improve the beamforming vector.
Transmit Diversity
Transmit diversity is a special type of multi-antenna transmission when the signal is transmitted from different antennas to achieve better spatial, angular and temporal diversities.
The most common transmit diversity consists of two transmit antennas. The signals from two or more transmit diversity antennas may be transmitted in different manners in terms of phases, amplitude, power, etc. This gives rise to different DL transmit diversity schemes. Some well-known schemes are:                Transmit beamforming open loop;        Transmit beamforming closed loop;        Switched antenna DL transmit diversity open loop;        Switched antenna DL transmit diversity closed loop; and        Space-time transmit diversity        
It should be noted that transmit diversity can be regarded as a special case of the well-known, multiple input multiple output (MIMO) transmission scheme, which can also be used in the DL. Embodiments described herein for DL transmit diversity can be extended or applied to any MIMO scheme, and vice versa.
In any MIMO or transmit diversity scheme, a set of parameters related to MIMO or DL transmit diversity are regularly adjusted by the BS. The objective is to ensure that the DL transmission incorporates the desired spatial, temporal or angular diversities. This may in turn improve DL coverage, reduce interference, increase downlink bit rate, enable BS to lower its transmitted power, to mention some advantages.
The MIMO or transmit diversity parameters may comprise any one or more of: antenna set, relative phase, relative amplitude, relative power, relative frequency, timing, absolute or total power of signals transmitted on transmit diversity branches, etc.
Furthermore, MIMO or any transmit diversity scheme can be used in any technology including LTE, WCDMA or GSM. For instance in LTE, the switched antenna uplink transmit diversity is standardized in LTE release 8.
Problems with Existing Solutions
One or more of the following problems may arise with existing solutions:                A UE performing measurements for positioning may not be aware of the antenna transmission scheme used at the BS transmitting radio signals used by the UE for positioning measurements. In particular the UE may have no information or very limited information about the antenna transmission scheme used in neighbor cells. Therefore the UE can make incorrect assumptions about the channel for different received signal samples which may in turn lead to degraded measurement quality (e.g., due to non-optimal accumulation). The positioning measurements (e.g., RSTD) are performed mostly on neighbor cells.        A BS transmitting radio signals for positioning may not be aware of whether all or some UEs support a certain transmission scheme for these signals and/or for positioning measurements. Such a BS may unaware of a quality of positioning measurements generated by a UE.        Positioning node assisting UE in performing positioning measurements by providing assistance data may not be aware of the transmission scheme used by a BS or UE's ability to support such transmission scheme for the radio signals used for positioning or for positioning measurements.        
The approaches described in this Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise expressly stated herein, the approaches described in this Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.